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Neuroendocrine axes
Thyrotropic
Somatotropic
Gonadotropic
Corticotropic
DA
GnRH
Brain
ACTH
LH/FSH
Pituitary
TSH
GH
Peripheral
glands
E2/T
11KT
T4/T3
IGF1
cortisol
Thyroid
Liver
Adrenals
Gonads
Target organs
Metamorphic changes
Figure 7 . Neuroendocrine axes involved in the control of secondary metamorphosis (silvering)
in eel. Silvering is triggered mainly by the gonatropic axis with some synergism with the
corticotropic axis. TSH=thyrotropin; T4=thyroxine; T3=triiodothyronine ; GH=growth
hormone; IGF=insulin-like growth factor; ACTH=corticotropin; LH=luteinizing hormone;
FSH=follicle-stimulating hormone; E2=estradiol; T=testosterone; 11KT=11-ketotestosterone.
DA=dopamine; GnRH=gonadotropin-releasing hormone.
levels. Moreover, exogenous sex steroids are able to induce peripheral
morphological changes observed during this process. This let us regard
eel silvering as a pubertal rather than a metamorphic event. The term
“prepuberty” was fi rst used by our group, as during eel silvering, puberty is
blocked at an early stage and further sexual maturation only occurs during
the reproductive migration. Other endocrine axes may participate in the
control of eel silvering. This is the case of somatotropic and corticotropic
axes acting in synergy with the gonadotropic axis (Fig. 7). Further studies
should aim at investigating the neuroendocrine interactions controlling
silvering, as well as the role of environmental and internal factors in the
mechanisms leading to the activation of the gonadotropic axis.
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